Units and Measurements

Without a standard unit system, there will not be any uniformity in measurement. Measurements are the only tools with which you can assert the validity of any theory. Hence, understanding the Units and Measurements is very important as it deals with the tool of comparison. Though a part of the school curriculum, particularly, the Physics syllabus for class 11, it comes into practice in our day-to-day life. A revision of the following class XI Physics chapter will give you an idea of the concepts dealt with in the chapter. 

What are Units and Measurements? 

When it comes to units and measurements, the very first thing is to understand what these terms denote. The comparison of any physical quantity in terms of the numerical value is called measurement. It provides a standard for everything in life. On the contrary, the standard by which quantities of similar nature are measured is called Units. The measurement is done based on internationally accepted units. 

Properties of Units

Listed below are some of the basic properties of a unit:

• It must be well-defined
• It should not vary with time
• It should not alter with changes in physical quantities such as pressure, temperature, height, weight, etc.

Types of Units

There are two types of units- Fundamental Units and Derived Units.

Fundamental Units

Another important concept in units and measurements is Fundamental Unis. The units for fundamental quantities that are defined and denoted by international systems of units are called fundamental units. There are seven fundamental units which have been tabulated below:

Quantity	Unit	Symbol
Distance or length	Metre	M
Time	Second	S
Electric current	Ampere	A
Mass or weight	Kilogram	Kg
Luminous intensity	Candela	cd
Temperature	Kelvin	K
Quantity or amount	Mole	mol

Supplementary Fundamental Units

Quantity	Unit	Symbol
Plane angle	Radian	rad
Solid angle	Steradian	Sr

Derived Units

It is a SI unit of measurement that consists of the combination of two or more of the seven fundamental units. There are many derived units in Physics. The examples are area, volume, speed, force, surface tension, pressure, latent heat, etc. 

Unit(s) Name	SI Unit	SI Unit Symbol	Expressed in SI Base Unit	Expressed in other SI units
Force, Weight	Newton	N	kg⋅m⋅s-2	–
Frequency	Hertz	Hz	s-1	–
Electric charge	Coulomb	C	s⋅A	–
Electric Potential (Voltage)	Volt	V	kg.m2.s-3.A-1	W/A
Inductance	Henry	H	kg.m2.s-2.A-2	Wb/A
Capacitance	Farad	F	kg−1.m−2.s4.A2	C/V
Resistance, Impedance, Reactance	Ohm	Ω	kg.m2.s−3.A−2	V/A
Electrical Conductance	Siemens	S	kg−1.m−2.s3.A2	Ω−1
Magnetic Flux	Weber	Wb	kg.m2.s−2.A−1	V⋅s
Magnetic Flux Density	Tesla	T	kg.s−2.A−1	Wb/m2
Energy, Work, Heat	Joule	J	kg.m2.s−2	N⋅m = Pa⋅m3
Power, Radiant Flux	Watt	W	kg.m2.s−3	J/s
Angle	Radian	rad	m.m−1	–
Radioactivity	Becquerel	Bq	s-1	–
Luminous Flux	Lumen	lm	cd	cd⋅sr

Check out: Deleted Syllabus for Class 11: All Subjects

Systems of Units

The units of measurement of physical quantities have evolved over centuries and decades. The old systems were not universal, owing to which there were discrepancies in measurement. These variations led to the evolution and adoption of a logical and universal International System of Units (SI) which is used in contemporary times. Here are the highly prevalent old and new systems of units:

Systems of Units	Unit of Length	Unit of Weight	Unit of Time
FPS	Foot	Pound	Second
CGS	Centimetre	Gram	Second
MKS	Metre	Kilogram	Second
SI	Metre	Kilogram	Second

Units and Measurements: Definitions of SI Base Units 

Now that you have understood the basic concepts related to units and measurements, let us go through the definition of different S.I units in detail. 

Metre

Metre is the length of the path travelled by light in a vacuum in a second and is expressed as m.s^–

Second

It is the SI unit of time defined in terms of radiation frequency at which the atoms of caesium change from one state to another. It is expressed as s. 

Candela

It is defined as the magnitude of the electromagnetic field and is the unit of luminous intensity. 

Ampere

It is the SI unit of electric current and represents the flow of one coulomb of electricity per second. 

Kilogram

It is the SI unit of mass. It is the quantitative measure of inertia and is expressed as kg.m².s^-1

Kelvin

It is defined as 1/273.16 (3.6609 x 10 ^-3) thermodynamic temperature of the triple point of water. 

Mole

It is defined as 6.02214076 × 10²³ of a substance. 

Units and Measurement Class 11: NCERT Chapter PDF

Latest Changes in SI Units

In 2019, on World Meteorology Day, 4 out of the 7 redefined S.I units came into existence. They were redefined by the General Conference on Weights and Measures taking various factors into consideration. Here are the important details you need to know about the changes in units and measurements.

• Kilogram will be based on Planck Constant. For this, a Kibble Balance will be used.
• Ampere will be denoted based on the electron’s charge.
• Avogadro Constant will be used to define Mole.
• Kelvin, which is the unit of temperature, will now be based on Boltzmann Constant.

Other Important Units and Measurements 

Apart from the aforementioned units and measurements, here are some others which you must understand. 

Astronomical unit – It is the average distance from the centre of the earth to the sun. 1 A. U = 1.495 x 10¹¹ m.

Light year – It refers to the distance travelled by light in one year. 1 light-year = 9.46 x 10¹⁵m

Parsec – It is used to measure large distances outside the solar system. 1 parsec = 3.3 light-years.

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